1. Field of the Invention
This invention involves a system and a method for determining local attenuation for ultrasonic imaging and in particular for rationalized gain control of ultrasonic images, especially those obtained by ultrasonic imaging of a region of a patient's body.
2. Description of the Related Art
Ultrasonic imaging of a body typically involves sending one or more ultrasonic signals into a region of a body and then sensing the relative strength of the signals returned as echoes from various portions of the region. All else being equal, signals return most strongly from portions of the region where the local change in acoustic impedance is greatest. The relative strengths of the return signals are then converted and processed and displayed in some form, for example, on a monitor, that represents an image of the scanned region.
The problem is, all other things are not equal. In the context of medical ultrasonic imaging, for example, several centimeters of tissue will often lie between the ultrasonic transducer and the region to be imaged. This tissue will typically greatly weaken (attenuate) the ultrasonic signals, in both directions, and may cause a return signal to appear weak even though it actually is an echo from an acoustically strongly reflecting region. Moreover, the problem is made worse if the tissue is not homogeneous; for example, a blood vessel may lie in the path of some of the ultrasonic signals but not of others. If one does not compensate for this uneven attenuation, the system might indicate structure in the region of interest when there is none, or fail to indicate structure of interest that is in the acoustic "shadow" of a non-homogeneous region between it and the transducer.
Yet another problem caused by attenuation is that the pulse-echo ultrasound signals have a large dynamic range. This makes them unsuitable for direct display on most conventional monitor screens, whose own dynamic range is normally too small.
One known way of dealing with the problem of attenuation is "time gain control" (TGC), in which the user manually enters a gain-versus-depth profile. The idea behind this method is that if one knows how deep (far from the transducer) the region of interest lies, and if one knows at least approximately the attenuation coefficients for the interrogated tissue and also the scanning geometry, for example, the focusing of the system, then one can employ time-gating techniques to compensate signals that arrive from different parts of the examined area. The gain can therefore be adjusted according to the depth of the region of interest.
One disadvantage of this known method is that an image frame is built up from several scanning beams, and the same gain profile is used for every scanning beam in the frame. Since the gain compensation cannot change from one beam to the next, this method can only match the proper attenuation profiles over a small number of beams. Variations in the geometry of the tissue over the field of view can therefore cause errors in the attenuation correction.
Because of the shortcomings of conventional, manual TGC, several other methods for rationalized gain control (RGC) have been proposed. In this context, "rationalized" means that the gain control depends on and is derived from the image itself rather than from a user-entered time relationship. Some of these determine a compensating gain function from an analysis of the echo intensities or the amplitude distribution of the picture elements ("pixels") in the image. In these methods, the gain compensation is thus indirect and does not result from a direct estimate of the attenuation. As such, they have unavoidable inaccuracies that degrade the ultimate image. Such methods are described in:
"Adaptive Time Gain Compensation for Ultrasonic Imaging," Ultrasound in Medicine and Biology, Vol. 18, No. 2, pp. 205-12, 1992, S. D. Pye, S. R. Wild, and W. N. McDicken;
"Ultrasonic Tissue Characterization Using Kurtosis," IEEE Trans. UFFC, Vol. 33, No. 3, pp. 273-79, 1986, R. Kuc; and
"Quantitative volume Backscatter Imaging," IEEE Trans. Sonics & Ultrasonics, Vol. 30, No. 1, pp. 26-36, M. O'Donnell.
In "Rational Gain Compensation for Attenuation in Cardiac Ultrasonography," Ultrasonic Imaging, Vol. 5, pp. 214-28, 1983, H. E. Melton, Jr., and D. J. Skorton derived a rationalized gain function based on a model of the diffraction, attenuation at the center frequency, and the frequency-dependent attenuation for cardiac ultrasonography. In this method, attenuation coefficients are approximated from the known attenuation of myocardium and blood through a detection circuit. The disadvantage of this method is that it's accuracy is limited to particular types of tested tissue.
A method for estimating attenuation as applied to TGC is described in "Application of stochastic analysis to ultrasonic echoes--Estimation of attenuation and tissue heterogeneity from peaks of echo envelope," J. Acoust. Soc. Amer., Vol. 79(2), pp. 526-34, 1986, P. He and J. F. Greenleaf. According to this method, the attenuation is computed from envelope peaks of each amplitude or "A" line by finding a minimum of a noise-to-signal ratio. Later, in "Acoustic attenuation estimation for soft tissue from ultrasound echo envelope peaks," IEEE Trans. Ultra. Ferroelec. Freq. Control, Vol. 36, No. 2, pp. 197-203, 1989, P. He proposed a split-spectrum processing method using the envelope peaks to estimate the attenuation in a broadband system. One reason to use split-spectrum processing is that it is equivalent to assuming a narrowband system and thereby to avoid the effect of frequency downshift found in a broadband system.
As its name implies, "frequency downshift" is the phenomenon that the return frequency spectrum of a broadband ultrasonic signal is shifted toward d.c. as it passes through the scanned tissue. This shift is non-linear, and failure to take it into account naturally causes inaccuracies in representation of the acoustic properties of the scanned region, that is, in the displayed image.
In "Attenuation measurement uncertainties caused by speckle statistics," J. Acoustic Soc. Amer., Vol. 80, pp. 727-34, 1986, K. J. Parker presented yet another method for estimating attenuation from decomposed radio-frequency signals using the Fast Fourier Transform (FFT). Attenuation is computed from a linear least-squares fit taken at the center frequency bin and then the values are averaged to reduce the variance of the computed attenuation coefficients. This decomposition process also acts as a narrowband system so that the linear least-squares model would be valid. As with many other known systems, this method in a sense makes the system fit the model rather than the other way around and, as a result, it introduces unavoidable inaccuracies.
Still another method for estimating attenuation is described in "Estimation of local attenuation from multiple views using compensated video signals," Acustica, Vol. 79, pp. 251-58, 1993, K. J. Peters, R. C. Waag, D. Dalecki and J. G. Mottley. In this method, one assumes a narrowband system and then estimates the local attenuation using envelope data. Even though this method assumes a narrowband system, the reference also discusses the effects of frequency downshift. To deal with this problem according to this method, one first estimates the attenuation without including the effects of downshift (by using linear least-squares data fitting to the decay model) and then updates the estimate by compensating for the attenuation-induced downshift. Nonetheless, this method attempts to model a non-linear system starting with a narrowband, linear assumption.
Yet another disadvantage of conventional systems is that they do not provide a measure of how accurate the local model is, in part because they do not provide a measurement of how heterogeneous the scanned tissue is. This means in turn that the user has no idea of how confident she can be in the results of the gain control.
In some applications, such as tissue studies, what is needed is a system and a method for accurately determining just what the local (that is, position-dependent) attenuation characteristics of the scanned region are. In other applications, what is needed is a system and a method that allow the user to control the gain by a direct estimation of the attenuation; the method and system should ideally not be restricted to being narrowband, and should preferably also provide a measure of tissue homogeneity.